Copper rod is a commercial commodity produced in vast quantities. A common type of furnace used in producing copper rod is the shaft furnace. Such a furnace is characterized by a generally vertical cylinder having gas burners at the bottom portion of the furnace which melt a copper charge introduced through the top of the furnace. One of the operating characteristics of a copper melting shaft furnace is a substantially continuous counter-current flow of the cold copper charge and the hot gas effluent from the burners.
The products which such furnaces are used to produce include tough pitch copper which contains an intentional amount of about 200 to 800 parts per million by weight (ppmw) of oxygen. The presence of small amounts of oxygen has been found to improve the electrical conductivity of the copper. However, too much oxygen can in itself reduce conductivity and other properties, and it is therefore important to control the oxygen content within narrow limits.
Occasionally, and for any number of reasons, the melting furnace must be shut down. The present practice in such instances is to terminate the flow of combustible gas to the burners, while maintaining a flow of air into the furnace for some period of time to solidify any molten copper in the furnace. However, during the cool down period, the molten copper and the hot copper charge absorb by diffusion processes an unacceptably high amount of oxygen. As a result, the already melted copper is no longer useful without some sort of refinement process, and the copper charge which will be melted when the furnace is restarted will produce a product containing excessively high oxygen levels. To accommodate such contamination, such high oxygen content copper is typically put aside and subsequently melted at a later time. This is an undesirable solution since the copper must be melted more than once and extra oxygen analyses of the melted copper must be performed.
It is known in the heat treatment of solid metal articles to use inert atmospheres, including nitrogen, to control (and preferably prevent) interactions between the article and the atmosphere. It is also known, especially in the casting of steel, to use nitrogen or other inert gas blankets for short time shielding of molten metal in ladles or tundishes.